How to Flatten Optional Values with Stream::flatMap and Java Streams?

Flattening Optional Values with Stream::flatMap and Java Streams

In Java 8, the Stream framework provides powerful APIs for concise code. However, certain scenarios, particularly involving Optionals, can be challenging to express succinctly.

Problem Statement

Consider a scenario where you have a list of objects and a method that resolves each object into an Optional. The goal is to map the objects to Optionals and then obtain the first non-empty value.

A straightforward approach would be to use things.stream().flatMap(this::resolve).findFirst(). However, flatMap requires returning a stream, and Optionals lack a stream() method.

Potential Solution

One solution involves using a multi-step operation:

<code class="java">things.stream()
    .map(this::resolve)
    .filter(Optional::isPresent)
    .map(Optional::get)
    .findFirst();</code>

While this approach works, it appears rather verbose.

Improved Solutions

Java 9 and later versions offer more elegant solutions:

Java 16:

<code class="java">Optional<other> result =
    things.stream()
          .map(this::resolve)
          .<other>mapMulti(Optional::ifPresent)
          .findFirst();</other></other></code>

Java 9:

<code class="java">Optional<other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(Optional::stream)
          .findFirst();</other></code>

Java 8 (Workaround):

Prior to Java 9, a helper function can be employed:

<code class="java">static <t> Stream<t> streamopt(Optional<t> opt) {
    return opt.isPresent() ? Stream.of(opt.get()) : Stream.empty();
}

Optional<other> result =
    things.stream()
          .flatMap(t -> streamopt(resolve(t)))
          .findFirst();</other></t></t></t></code>

These enhanced solutions provide more concise and efficient ways to handle Optionals within stream operations.

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